The present invention relates to a gas-cooled single-chamber type heat-treating furnace for heat-treating metallic materials such as steel parts and a gas cooling method in the furnace.
The gas-cooled single-chamber type heat-treating furnace is known as a furnace for cooling metallic materials by the forced circulation of cooling gas after heating the metallic materials.
In this gas-cooled single-chamber type heat-treating furnace, cooling gas vents opened and closed by doors are provided on each of mutually opposed walls of an inner chamber, namely a processing room provided in a casing. The cooling gas vents are closed by the doors when the metallic materials are heated, and then the metallic materials charged into the inner chamber are heated by a heater provided in the inner chamber. When the heated metallic materials are cooled, the cooling gas vents are opened, and then cooling gas cooled by a cooler is supplied into the inner chamber from one cooling gas vent by means of a circulating fan provided in the casing and is directed to the circulating fan from the other cooling gas vent, whereby the metallic materials are cooled during the circulation of this cooling gas.
The cooling gas vents have a large opening area so as to supply a sufficient amount of cooling gas to the metallic materials in the inner chamber, and are opened and closed by sliding or lifting doors.
In the above conventional gas-cooled single-chamber type heat-treating furnace, each of the cooling gas vents has simply an opening. Therefore, a flow of the cooling gas in the inner chamber during cooling is inclined to concentrate on a center portion of the cooling gas vents, so that the metallic materials cannot be cooled uniformly.
Furthermore, in case a sliding doors are employed which move in parallel along the cooling gas vents, it is required to minimize a clearance between the doors and the inner chamber to enhance a sealing performance of the doors during the inner chamber is closed. However, if this clearance is made too small, the inner chamber does not operate properly due to a slight thermal strain of the doors or the inner chamber, whereby good sealing performance cannot be maintained for a long time. As a result, such a problem arises that the temperature distribution in the inner chamber becomes uneven during heating.
In case the lifting doors are employed, enough sealing performance cannot be maintained due to a thermal strain of the cooling gas vents of the inner chamber. This also causes a problem of uneven temperature distribution in the inner chamber.
The present invention has for its object to provide a gas-cooled single-chamber type heat-treating furnace in which a flow of the cooling gas in the inner chamber during cooling is not inclined to concentrate on the center portion of the cooling gas vents. In addition, the present invention has for its object to provide a gas-cooled single-chamber type heat-treating furnace in which good sealing performance is maintained between the doors and the inner chamber.
In the meantime, a gas cooling method has been known as a cooling method in the heat treatment of metallic materials. Furthermore, with respect to a cooling treatment, such a cooling method has been known that, for example, a metallic material kept at a hardening temperature is rapidly cooled in the critical temperature range to a temperature just above the martensitic transformation starting temperature and is slowly cooled, conversely, in the dangerous temperature range at or below the martensitic transformation starting temperature.
The gas cooling method mentioned above is roughly classified into an internal circulation type (in which a circulating fan is provided inside the furnace) and an external circulation type (in which a circulating blower is provided outside the furnace). In any one of the above types metallic materials of different classes or shapes can be heat-treated in the same furnace. Therefore, according to the above-mentioned gas cooling method, cooling based on a proper temperature pattern corresponding to a class or a shape of each metallic material becomes necessary so as to reduce a strain of the metallic material and achieve an expected object.
Furthermore, a forced convection cooling method is known, in which a gas density of a circulated atmosphere changes in response to a temperature change in the circulated atmosphere whereby a heat transfer coefficient changes. That is, a cooling effect lowers under a condition of a constant number of revolutions of a fan because a gas density is low when atmosphere temperature is high during an initial period of cooling. In order to eliminate this problem, there is proposed a method for improving the cooling effect by running the circulating fan or the circulating blower at a high speed in response to a change in the furnace atmosphere temperature or metallic material temperature in the furnace (Japanese Patent Laid-open Publication No.52-119408).
In case the forced convection cooling method, such a problem arises that cooling in response to a preset cooling curve cannot be achieved because only the number of revolutions of the fan is changed directly on the basis of the furnace atmosphere temperature or the metallic material temperature in the furnace.
In addition, a capacity of the drive motor of the circulating fan in the internal circulation type or that of the circulating blower in the external circulation type is determined in consideration of a furnace capacity, efficiency and soon. Thus, such a problem arises that the drive motor may run over its rated number of revolutions in a specific cooling state, whereby a risk of burning of the drive motor occurs.
The present invention, therefore, has for its object to provide a cooling method of a metallic material, in which in order to solve above mentioned problems, the drive motor exhibits a maximum cooling capacity by running the drive motor at an allowable critical power when a preset cooling speed is higher than an actual cooling speed, while otherwise the cooling speed of the metallic material is adjusted through control of a number of revolutions of the drive motor such that the furnace atmosphere temperature or the temperature of the metallic material in the furnace will change at the preset cooling speed.
In order to achieve the above objects, according to the present invention, there is provided a gas-cooled single-chamber type heat-treating furnace in which cooling gas vents opened and closed by doors are provided on each of mutually opposed walls of an inner chamber forming a processing room and a cooling gas is circulated by opening the cooling gas vents during gas cooling, wherein the cooling gas vents of the inner chamber are provided with lattice-shaped flow uniforming members of heat-resisting materials.
Thus, the cooling gas vents of the inner chamber are provided with lattice-shaped uniforming members, thereby the flow of an incoming gas into the inner chamber and an outgoing gas from the inner chamber are controlled, thereby resulting in reducing the flow of cooling gas in the inner chamber inclined to concentrate on the center of the cooling gas vents in the inner chamber, so that the metallic materials can be cooled uniformly.
Furthermore, in the gas-cooled single-chamber type heat-treating furnace according to the present invention, the cooling gas vents are at an upper portion and a lower portion of the inner chamber and the doors are of a lifting type, and a pressing contact portion between a peripheral portion of each door and the inner chamber has a structure in which a projection is held in engagement with a recess.
Thus, since the pressing contact portion formed at peripheral portions of the each door and the each cooling gas vent of the inner chamber has a structure in which a projection is held in engagement with a recess, sealing performance is secured even if a clearance occurs between a tip portion of the projection and the recess due to thermal expansion etc., and the temperature distribution in the inner chamber is not disturbed.
Meanwhile, it is preferable to make the lattice-shaped flow uniforming members of thin plates of carbon graphite fiber composite.
By making the lattice-shaped flow uniforming members of thin plates of carbon graphite fiber composite, the lattice-shaped flow uniforming members have a small volume of heat storage and great strength. Therefore, the responsiveness during heating and cooling is never damaged, and an effect that a flow of great volume of cooling gas can be obtained without any obstruction.
Furthermore, in order to achieve the above objects, according to the present invention, there is provided a gas cooling method in a gas-cooled single-chamber type heat-treating furnace in which a metallic material heated to a predetermined temperature is cooled by forced convection, wherein a number of revolutions of a drive motor of a circulating fan or a circulating blower is controlled based on a difference between a preset cooling curve and an atmosphere temperature in an inner chamber or a metallic material temperature obtained by comparing the atmosphere temperature or the metallic material temperature with the preset cooling curve; wherein the drive motor is kept to run at its critical output even if a load increases due to a temperature change when an output of the drive motor reaches the critical output.
Thus, since the number of revolutions of the drive motor of the circulating fan or circulating blower is controlled by a temperature feedback and an output feedback, the maximum cooling capacity can be achieved during rapid cooling, while a cooling process corresponding to the preset cooling curve is performed during slow cooling.